1. Field of the Invention
This invention relates to a piston ring assembly for an automotive internal combustion engine.
2. Description of the Prior Art
Generally, in an automotive internal combustion engine, a piston, which constitutes part of a combustion chamber, performs the important functions of effecting intake, compression and exhaust, while at the same time transmitting an explosion force, which results from combustion of a fuel-air mixture in the combustion chamber, to a crank shaft through a connecting rod.
In order that the intake, compression and exhaust may be carried out under a satisfactory condition and the explosion energy may be effectively transmitted, it is required that a completely sealed space be defined by the piston and cylinder of the engine.
To meet the foregoing requirement, there has heretofore been proposed such an arrangement as shown in FIG. 1 of the accompanying drawings, wherein a first compression ring 2 and a second compression ring 3 are provided on the outer circumferential surface of the head portion of a piston 1 to thereby seal the sliding gap between the outer circumferential surface of the piston 1 and the inner surface 4 of the associated cylinder; and an oil ring 5 is also provided which is adapted to control formation of lubricating oil film on the cylinder inner surface 4 to thereby make smooth the movement of the first and second compression rings 2 and 3, while at the same time minimizing consumption of the lubricating oil. In FIG. 1, indicated at 6, 7 and 8 are grooves in which the piston rings 2, 3 and 5 are fitted respectively.
The inventors have recently made various experiments and investigations to enhance the sealing ability of a piston ring with a view to reducing the quantity of blow-by gas to thereby realize lower fuel consumption and prevent early degradation of lubricating oil. As a result, it has been confirmed that with an automotive internal combustion engine, it is possible to sufficiently reduce the quantity of blow-by gas by employing, as the aforementioned first compression ring 2, a piston ring 2 provided with double-stepped mating ends such as shown at 2A in FIGS. 2(A) and (B) which have conventionally been used with a large-type marine engine operating in a lower speed range than an automotive engine. (Refer to "Automotive Mechanics", pages 80 to 81, 1970, published by McGraw-Hill, Inc.)
As the pressure in the combustion chamber builds up through compression stroke into expansion stroke, the piston ring 2 having the double-stepped mating ends 2A is pressed against the lower surface of the ring groove 6, and under such a condition, a fuel-gas mixture or burnt gas, which has entered a space behind the ring 2 through the upper gap or upper mating end gap (a) of the ring 2, is prevented from leaking toward the lower mating end gap (b) thereof. Thus, the sealing ability of the ring 2 is further enhanced, so that the quantity of blow-by gas is reduced accordingly.
With the piston 1 using the first compression ring 2 having the double-stepped mating ends 2A, however, since the quantity of blow-by gas is reduced as mentioned above, it is not possible to prevent lubricating oil from ascending toward the combustion chamber with the aid of blow-by gas as in an arrangement wherein the first compression ring 2 is not provided with the double-stepped mating ends. Moreover, since the engine of interest is of a high speed type, the pressure occurring at the sliding gap between the first and second compression rings at expansion stroke of the engine, turns out much lower than in the case of the last-mentioned arrangement; thus, at the former half of the expansion stroke, the second compression ring 3 tends to flutter, instead of closely contacting the lower surface of the ring groove 7, when the engine is operating in the high-speed range with which an increased inertia force is imparted to the ring 3. For these reasons, there is the tendency that the quantity of oil consumption is increased especially when the engine is operating in the high-speed range, as will be seen from FIG. 3. In FIG. 3, the solid curve (A) indicates the engine speed-oil consumption characteristic for the case where the first compression ring 2 is provided with the double-spepped mating ends 2A, and the dashed curve (B) shows the engine speed - oil consumption characteristic for the case where the first compression ring 2 is not provided with the double-stepped mating ends.